Flux composition and a centrifugal casting process using the same

ABSTRACT

There are provided a flux of a novel composition adapted for use in a centrifugal casting process for the production of a composite roll and the process comprising the step of adding the flux at a specific stage thereof. This flux contains, in weight percent, from 5 to 20% a metallic element such as aluminum which causes an exothermic reaction with the melt when in contact, thus aiding in the heat generating on the innermost surface of the melt to define the shell portion of the roll during the casting. This improves the characteristic of the temperature distribution gradient over its cross-section taken radially of the shell portion of the roll, thereby precluding the cause of the formation of cracks or other defects which tend to appear in the form of a circumferential layer in the formed roll but at a position close to its outermost surface in its cross-section.

United States Patent [191 Yokota Mar. 18, 1975 [75] Inventor: ReizoYokota, Hirakata, Japan 173] Assignee: Yodogawa Steel Works Ltd.,

Osaka-shi, Osaka-fu, Japan Filed: Dec. 15, 1972 21 Appl. No.: 315,499

[52] US. Cl 75/94, 75/27, 75/53 {51] Int. Cl C22b 9/10 [58] Field ofSearch 75/94, 27, 58, 53, 55, 75/57; 148/26 [56} References Cited UNITEDSTATES PATENTS 1358.328 6/1966 Goss 75/57 3.259972 7/1966 Long 75/273.331304 5/1967 Snow 75/53 333L680 7/1967 Leupold 75/53 PrimaryExaminer-Peter D. Rosenberg Attorney, Agent, or FirmLane, Aitken, Dunner8t Ziems [57] ABSTRACT There are provided a flux of a novel compositionadapted for use in a centrifugal casting process for the production of acomposite roll and the process comprising the step of adding the flux ata specific stage thereof. This flux contains, in weight percent, from 5to 20% a metallic element such as aluminurn which causes an exothermicreaction with the melt when in contact, thus aiding in the heatgenerating on the innermost surface of the melt to define the shellportion of the roll during the casting. This improves the characteristicof the temperature distribution gradient over its cross-section takenradially of the shell portion of the roll, thereby precluding the causeof the formation of cracks or other defects which tend to appear in theform of a circumferential layer in the formed roll but at a positionclose to its outermost surface in its crosssection.

1 Claim, 4 Drawing Figures PATENTED 81975 3,871,875

SHLU 1 OF 2 temperature position 6 position 5 distance PATENTEUHARI 8 msSHEET E OF 2 T R A R m. Du P FLUX COMPOSITION AND A CENTRIFUGAL CASTINGPROCESS USING THE SAME BACKGROUND OF THE INVENTION This inventionrelates to a flux composition and a centrifugal casting process for usein the production of a composite roll, which comprises the step ofadding said flux at a specific stage of the casting, and moreparticularly to a flux which leads to an exothermic reaction with thesteel melt when in contact.

In conventional, horizontal type centrifugal casting process for use inthe production of a composite roll, a cylindrical casting mold havingopen ends is placed in a horizontal direction and rotated. Then, alloymelt is introduced therein to form the outer layer or shell portion of aroll, while charging thereafter an oxidation-preventive flux to coverthe radially innermost surface of the melt to define the shell portionof the roll. After the solidification of the shell portion, the rotationof the casting mold is ceased, then one end of the mold is closed with aconventional means, followed by the erection of the casting mold on asuitable base, with said closed end placed to form the bottom thereof.Subsequently. alloy melt is poured into said mold from top or bottom tofill the hollow portion or core portion defined by said shell portion ofthe roll. At this time, the flux coating covering the radially innermostsurface of the shell portion will float on the top surface of the meltwhich has been cast. Naturally, the addition of the flux is directed toimproving the surface condition of the radially innermost surface of theshell portion, thereby providing satisfactory metallurgical bondingbetween the shell and core portions of the roll.

However. this process suffers from the inherent disadvantages resultingfrom the solidification mode, that is to say, from the unsuitabletemperature distribution or gradient throughout its cross section takenradially of said shell portion.

More specifically, referring to the solidification phase of the shellportion after addition of the flux, the cooling of the shell portionfrom the radially outermost surface thereof, i.e., the cooling throughthe metal mold, with which the shell portion is in contact, is greaterthan that from the radially innermost surface of the shell portion whichis covered with the flux layer. This is partly because the thermalconductivity of the flux is appreciably less than that of the metal moldand partly because of the bulky mass of the metal mold which ismaintained at a considerably lower temperature as compared with that ofthe melt. This results in the formation of a temperature distribution orgradient having the peak whose position is biased towards the innermostsurface of the shell. In other words, this portion or layer biasedtoward the innermost surface of the shell extends circumferentially ofthe shell portion and is the layer which will be solidified last. Insuch a location of the shell where solidificiation occurs last, theretends to take place segregation of impurities such as phosphide, sulfideand the like, and the formation or the growth of large crystal grainswhich produce adverse effects in the mechanical properties of the roll.

The metallurgical structure depends on the cooling rate. For instance,when a case alloy such as high alloy chilled cast iron, alloy grain castiron, or plain chilled cast iron is cast to form the shell portion ofthe roll, then a normal structure is not obtained as expected in theaforesaid layer where solidification last takes place.

The result is abnormal and aggregating structure in precipitatedgraphites of a considerably great amount.

The phenomena such as the segregation of the impurities, the growth ofthe crystal grains and the segregation of the graphite are even moreevident in the shell portion of a roll of a considerably great diameter,and such phenomena are likely to occur along the circumferentialdirection of the shell therein but in a position radially adjacent to orin the vicinity of the radially innermost surface of the shell portionwhen solidified.

Since the shell portion of the roll is important in that it directlycontacts the material to be rolled and thus is frequently subjected tothermal and mechanical stresses, such a defect assuming the form of thincircumferential layer in the shell portion is critical, because itpresents the cause for the .fatal failure of the roll during itsservice.

Hitherto, to avoid such a drawback, for instance, a burner is introducedinto the casting mold, after casting shell portion, to heat theinnermost surface thereof in an attempt to adjust or improve theaforesaid temperature distribution or gradient over its cross-section.In other words, the peak of the temperature gradient is shifted inwardlyto give a smooth curvature of the temperature distribution or gradientwhich will be described in more detail hereinafter.

Nevertheless, such an operation using a burner necessitates providingcomplicated devices and is timeconsuming, thus only partially solvingthe problem.

Accordingly, it is an object of the present invention to provide a fiuxof a novel composition which improves the temperature distribution orgradient over the cross-section of the shell portion during thesolidification phase thereof, while preventing re-oxidation of theradially innermost surface of the melt to define the shell portion,which has been spoiled due to oxidation before the flux is applied andthus purifying said surface.

It is another object of the present invention to provide a centrifugalcasting process for use in the production of a composite roll, saidprocess comprising the addition of aforesaid flux at a specific stage ofthe process.

SUMMARY OF THE INVENTION In accordance with the present invention, thereis provided a flux containing, in weight percent, from 5 to 20% of ametallic element such as aluminum which causes an exothermic reactionwith the alloy melt when in contact, thus leading to the head generationon the radially innermost surface of the melt of the shell portion of acomposite roll. This avoids the use of a conventional burner andprovides satisfactory temperature distribution or gradient throughoutthe cross-section of the shell portion.

Furthermore, according to the invention, the flux consists, for example,of from 30 to 45% by weight of a mixture of borax. and silica sand, from5 to 20% by weight of aluminum in the form of powder and the balancebeing soda compounds, fluoride, etc. as fusing agents. In operation, theflux should be scattered over the radially innermost surface of the meltto define the shell portion, immediately after the casting of the meltfor the shell portion or at least prior to the commencement of thesolidification of the shell portion, and then the melt to define thecore portion thereof is cast,

thereby improving the metallurgical bonding between the shell portionand the core portion.

However, it is preferably to add the flux, immediately after thecompletion of the casting of the melt in an attempt to retard orsuppress the cooling of the innermost surface of the shell portion dueto the exothermic reaction and the heat retaining capability of theflux, thereby smoothing or blunting the peak configuration of the curverepresenting the temperature distribution, while shifting the peaktoward the innermost surface of the shell portion.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a cross-sectional view of acasting mold and shell portion, after the casting of the shell portionhas been completed,

FIG. 2 is a plot showing the temperature distribution or gradient overthe cross-section taken along the line AA of FIG. 1;

FIG. 3 is a microscopic view of the structure of the shell portion whichhas been cast according to the conventional centrifugal casting process;and

FIG. 4 is a microscopic view of the structure of the shell portion whichhas been cast according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The flux of the presentinvention has a composition consisting essentially of. in weightpercent, from to of a metallic element such as aluminum which leads toexothermic reaction when in contact with a melt, from to 45% mixture ofborax and silica sand and from 65 to sodium compound such as soda ashand/or fluoride such as fluorite.

i. The amount of aluminum can vary between 5 and 20%. Aluminum in theform of metal powder affords a strong deoxidizing effect and forms A1 0with a great amount of heat which generates according to the followingreaction formula;

4(Al) 30 Z(Al I520 Kcal/mol.

This results in the deoxidation ol the iron oxide exist ing prior tocharging of the flux on the radially innermost surface of the melt todefine the shell portion of a roll. while resulting in a great amount ofheat due to the reaction with oxygen, thus raising the temperature ofthe flux itself and at the same time heating the radially innermostsurface of the shell portion to thereby improve the temperaturedistribution or gradient over the cross-section thereof. THe A1 0 thusproduced will be mixed with the additives such as borax, silica sand.sodium compound and fluoride which form a thin coating layer over theradially innermost surface of the shell. serving theoxidation-preventive purpose. Amounts of aluminum less than 5% reducethe exothermic effect. while amounts of aluminum greater than 20%increase the viscosity of the fused additives due to the increase inamount ofthe A1 0 having a high melting point. resulting in theincomplete formation of the thin coating layer of the flux.

ii. A mixture of borax and silica sand is present in the range from 30to This mixture serves to give noncrystalline viscosity (vitreousviscosity) to the flux in a wide temperature range. such as from 800C tol,3()()('. The optimum level of the mixture is in the range of 30 to45'?! from the viscosity standpoint. However, best results are obtainedby decreasing the amount ofsilica sand with increasing amounts ofaluminum.

iii. The amount of sodium compound. fluoride and the like is between 65and 35%. the sodium compound such as soda ash serving to providefluidity to the flux at a lower temperature, while the fluoride such asfluorite serves to provide fluidity to the flux at a relatively highertemperature and enhance the solubility of oxide. However, the amounts ofthe sodium compound and/or fluoride should depend on the amounts ofaluminum and borax and silica sand added for adjustment of the fluidityof the flux.

Referring now to FIG. 1, shown at (l) is a wall of a rotating castingmetal mold, at (2) the shell portion of a roll and at (3) the coreportion of the composite roll. Shown at (4) is a coating of a flux whichhas been supplied in the form of powder over the radially innermostsurface of the melt to define the shell portion of the composite roll.Numerals (6) and (5) represent intersections of the line A-A with theinnermost surface of the mold and the innermost surface ofthe shellportion. respectively.

FIG. 2 shows a plot of the temperature distribution or gradient over thecross-section taken along the line AA of FIG. 1, in which the peak inthe temperature is biased towards the ordinate side, appearingimmediately above the position (5), then gradually lowered towards theordinate of the position (6). The solid line represents the temperaturedistribution created according to the convention centrifugal castingprocess, while the broken line represents the temperature distributioncreated according to that of the present invention which gives a smoothcurvature particularly at the peak which should have existed in theconventional castings.

As has been described. the peak in the temperature means the position inthe shell portion whose cooling rate is last retarded. The broken line(7) in FIG. 1 corresponds to the peak shown in FIG. 2, where impuritiessuch as phosphide, sulfide and the like tend to segregate in the shellportion of the roll.

EXAMPLE 1 A composite roll was cast having a shell portion of 650 mm inouter diameter, 1.600 mm in length and 50 mm in wall thickness. The meltused for the shell portion was high alloy grain chilled cast iron, whilethe melt used for the core portion was high'tensile cast iron. Thecomposition of the flux used consists, in weight percent, of 40% borax,5% silica sand, 35% soda ash, 10% fluorite and 10% aluminum in the formof powder.

FIG. 3 and FIG. 4 compare the structures of the shell portions castaccording to the conventional process and present inventionrespectively.

Both photographs were taken at a magnification of 50. the samples beingetched with 3% alcohol nitrate for 15 to 20 seconds.

The material used for the shell portion was high alloy grain iron.

FIG. 3, the segregated layer ol graphite was observed running along thediagonal direction of the picture. while the direction of the segregatedlayers of graphites are shown normal to the surface of the photograph.

1. A flux composition adapted for use in the Centrifugal casting of acomposite roll from a metal melt said composite roll having a coreportion and a shell portion directly bonded to said core portion, saidflux consisting essentially of, in weight percent of the total. from 5to 20% of a metallic element selected from aluminum, silicon andmagnesium, from 30 to 45% of a mixture of borax and silica sand and from65 to 35% of a fluidizing agent selected from sodium compounds,fluorides and mixtures thereof.

1. A FLUX COMPOSITION ADAPTED FOR USE IN THE CENTRIFUGAL CASTING OF ACOMPOSITE ROLL FROM A METAL MELT, SAID COMPOSITE ROLL HAVING A COREPORTION AND A SHELL PORTION DIRECTLY BONDED TO SAID CORE PORTION, SAIDFLUX CONSISTING ESSENTIALLY OF, IN WEIGHT PERCENT OF THE TOTAL, FROM 5TO 20% OF A METALLIC ELEMENT SELECTED FROM ALUMINUM, SILICON ANDMAGNESIUM, FROM 30 TO 45% OF A MIXTURE OF BORAX AND SILICA SAND AND FROM65 TO 35%OF A FLUIDIZING AGENT SELECTED FROM SODIUM COMPOUNDS, FLUORIDESAND MIXTURES THEREOF.